I went to about:config (yes, there be dragons!) and looked up useragent. I noticed that the list of preference names included Preference Name: general.useragent.override.facebook.com Value: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.9) Gecko/20100101 Firefox/45.9 (Pale Moon). I also noticed a more reasonable one in the general.useragent.override.google.com preference (Mozilla/5.0 (X11; Linux x86_64; rv:52.9) Gecko/20100101 Goanna/4.1 Firefox/52.9 PaleMoon/28.1.0). I chose to use the more reasonable one.